C# saving any JSON string as a Firestore document

I'm trying to retrieve a document from Firestore, edit it as a plain-text JSON, and then reupload the changes. I'm using JSON.net to deserialize the string to Dictionary<string, object>, and then pass it on to SetAsync();:

public static async void SaveDocumentDataAsync(string documentAccessPath, string documentData)
{
     DocumentReference documentRef = firestoreDb.Document(documentAccessPath);
     Dictionary<string, object> document = JsonConvert.DeserializeObject<Dictionary<string, object>>(documentData);
     await documentRef.SetAsync(document);
}

This works perfectly if the JSON object is simple, like:

{
    "foo": "bar"
}

But if it has nested elements, everything above the root properties gets saved as an empty array (the keys are all correct, though):

{
    "foo": "bar"
    "baz": {
         "property1": 6,          // will be saved to Firestore as "property1": [],
         "property2": ["text", 5] // will be saved to Firestore as "property2": [],
    }

}

The closest I got to some sort of solution is using Dictionary<string, Dictionary<string, object>> but this only works for root and second level properties, everything else is saved as an empty array again.

Is there a simple way, or any way, to just translate JObject to Dictionaries with only the key and the actual values in them?

I've been trying to find something for a good while but none of the other solutions worked, and I can't make custom classes for the documents, because their contents can be anything, no specific structure.